With the increasing cost of gas and fuel oil, in combination with an ever increasing social consciousness directed to conservation and recycling, further attention is being directed to alternate fuel sources such as wood and other solid/semi-solid combustibles or combustible residues. A resurgence of interest and attention is being given to biofuels, biofuel combustion processes and biofuel combustion devices.
“Biofuel” is a term generally understood to embrace any fuel derived from biomass, namely, recently living organisms or their metabolic byproducts. Agricultural products specifically grown for use as biofuels include, among other things, corn, soybeans, flaxseed, rapeseed and hemp. Furthermore, biodegradable outputs from industry, agriculture, forestry, and households, e.g., straw, timber, manure, sewage and food leftovers, can also be used to produce “bioenergy.”
Heaters or stoves, more generally, “heating appliances,” for burning biofuels are known to provide acceptable alternative heat sources for conventional heating units such as gas, electric and oil furnaces. Biomass pellets of a variety of compositions are well-known fuel sources, as are cereal grains such as corn and wheat, to name but a few.
While many perceive pelletized fuel sources as being especially advantageous due to size uniformity and low moisture content, efficient energy producing combustion nonetheless requires attentive regulation of a variety of combustion parameters, for example, and without limitation, draft regulation, backfire prevention, thorough fuel conversion, ash management/conditioning and exhaust flue temperature. In light of, among other considerations, increasing costs for pelletized biofuels due to increasing, and in some places, unmet demands for same, it is believed especially advantageous and desirable to utilize raw biomass fuel sources such as grains, which generally are readily available, and favorably priced on a per unit of energy produced basis.
Shortcomings and/or challenges associated with efficient biofuel combustion have been, and continue to be well documented. For instance, U.S. Pat. No. 7,004,084 (Anderson et al.), the contents of which are incorporated by reference, identifies a variety of challenges and, as the case may be, heretofore known approaches to those challenges, namely those of: fuel delivery (1:36 et seq.); initial fuel ignition and startup (1:60 et seq.); clinker formation (2:34 et seq.); and, thermal operational optimization (2:45 et seq.). Although incremental improvements have arguably been made, there remains ample room for improvement (see, e.g., copending U.S. Pat. application Ser. No. 11/550,494, filed Oct. 16, 2006, entitled “Apparatus for Combustion of Biofuels,” incorporated herein by reference in its entirety.
Heretofore known combustion appliances are generally limited in their dynamic range, i.e., their ability to run at both very high and very low heat outputs, even when utilizing a single, uniform fuel source, e.g., a pelletized biomass, let alone when the biofuel may be variable from one heating event to another. Furthermore, heretofore known combustion appliances require initial set-up or setting of combustion regulating parameters, whether a factory or “on-site” setting, and commonly require periodic resetting or adjustment of the initially selected parameters.
Temperature of the air streams within the appliance are an important operational consideration or parameter. In biomass burning devices or appliances, one of the most important and pervasive factors or parameters in establishing and/or maintaining combustion efficiency is the hot air exhaust temperature.
As should be readily appreciated, heat can enter the room either via the heated convection air stream, through passive convection directly from heated stove surfaces, or through radiation from heated stove surfaces. Heat carried to the outside by the exhaust gas is for all practical purposes wasted. Because of this fact, lowering the temperature of exhaust gases will improve the stove efficiency. There is however a limit to this approach.
As is generally well known, as exhaust gas temperatures fall below about 250° F., condensation can occur in the exhaust flue. This condensation is usually highly acidic which produces rapid deterioration of the flue. To avoid this problem, while still maintaining the best feasible efficiency, the exhaust temperature should be controlled to be as close to about 250° F. as possible. Thus, for a given burn rate, which is primarily and advantageously controlled by the fuel feed rate, two controllable parameters are available for regulation in furtherance of managing the exhaust temperature, namely, the air flow rates associated with combustion and convection/recirculation.
In light of the foregoing considerations, and relative to the present state of the art and improvements or improved features in and of the new, i.e., last generation of biofuel heaters/heating appliances, there nonetheless remains great room for device and process control improvement, especially in the arena of non-industrial applications. It remains highly desirable to provide an apparatus which can, for, all practical purposes, efficiently operate with no onsite calibration, modification, alteration, upgrade, retrofit, etc., and further still, an apparatus which can readily process a variety of biomass feed stocks as fuel, i.e., either or any of pelletized biomass, semi-processed biomass, or raw biomass, separately, or in combination. Furthermore, it remains desirable and advantageous to more efficiently handle fuel distribution and management, as well and improve upon heretofore known ash conditioning or management techniques. Finally, there remains a need to eliminate ignition and start-up shortcomings; to provide and/or support a combustion process which is less dependent upon the plurality of heretofore adjustments in relation to one or more of fuel feed type, character or quality, fuel feed rate, and/or combustion air dynamic, flow and character; and, to provide one or more dynamic process/operational controls in furtherance of optimal/maximum thermal efficiency for such appliances under a variety of conditions and/or designated parameters.